The viscosity of dilute aqueous solutions of 2-methyl-2-propanol (tert-butyl alcohol, TBA) has been measured between (-10 and 25) degrees C with a falling-body viscometer to a maximum pressure of 388 MPa. The expanded uncertainty is estimated at +/- 2 %. Comparison is made with earlier measurements of the intradiffusion coefficient of water in TBA solutions under similar conditions. (K. R. Harris and P. J. Newitt, J. Phys. Chem. A 1999, 103, 6508). At the higher concentrations, the fluidity, like the water self-diffusion coefficient, is linear in the solution molar volume. As the solutions become more dilute and the molar volume decreases, fluidity maxima appear, first at the lower temperatures, then gradually extending to higher temperatures. The positions of the maxima vary with composition and temperature. Relative to the values at atmospheric pressure, the maxima in the more dilute solutions (x = 0.01, 0.025) are greater than those for pure water under the same conditions and shifted to slightly higher pressures. Consistent with conclusions drawn from high-pressure neutron-scattering studies (V. Calandrini et al. J. Phys.: Condens. Matter 2006, 18, S2363), it appears that the effect of the dissolved solute is similar to that of a temperature drop of 5 K in pure water in terms of the effect on the relative viscosity.